1. Field of the Invention
The present invention relates to ink jet printing, and more particularly, to the structure of nozzle-defining channels included in ink jet printheads.
2. Description of Related Art
In ink jet printing, a printhead is usually provided having one or more ink-filled channels communicating with an ink supply chamber at one end and having an opening at the opposite end, referred to as a nozzle. These printheads form images on a recording medium such as paper by expelling droplets of ink from the nozzles onto the recording medium. The ink forms a meniscus at each nozzle prior to being expelled in the form of a droplet. After a droplet is expelled, additional ink surges to the nozzle to reform the meniscus. An important property of a high quality printhead array is good jet directionality. This ensures that ink droplets can be placed precisely where desired on the print document. Poor jet directional accuracy leads to the generation of deformed characters and visually objectionable banding in half tone pictorial images.
A major source of ink jet misdirection is associated with improper wetting of the front face of the printhead which contains the array of nozzles. One factor which adversely affects jet directional accuracy is the interaction of ink accumulating on the front face of the printhead array with the ejected droplets. Ink may accumulate on the printhead face either from overflow during the refill surge of ink or from the spatter of small satellite droplets during the process of expelling droplets from the printhead. When the accumulating ink on the front face makes contact with ink in the channel (and in particular with the ink meniscus at the nozzle orifice) it distorts the ink meniscus resulting in an imbalance of the forces acting on the egressing droplet which in turn leads to jet misdirection. This wetting phenomenon becomes troublesome after extensive use as the array face oxidizes or becomes covered with a dried ink film. This leads to a gradual deterioration of the image quality that the printhead is capable of generating. In order to retain good ink jet directionality, wetting of the front face desirably is suppressed. Alternatively, if wetting could be controlled in a predictable, uniform manner, jet misdirection would not be a problem. However, uniform wetting is difficult to achieve and maintain.
In thermal ink jet printing, a thermal energy generator, usually a resistor, is located in the channels near the nozzles a predetermined distance therefrom. The resistors are individually addressed with a current pulse to momentarily vaporize the ink and form a bubble which expels an ink droplet. As the bubble grows, the ink bulges from the nozzle and is contained by the surface tension of the ink as a meniscus. The rapidly expanding vapor bubble pushes the column of ink filling the channel towards the nozzle. At the end of the current pulse the heater rapidly cools and the vapor bubble begins to collapse. However, because of inertia, most of the column of ink that received an impulse from the exploding bubble continues its forward motion and is ejected from the nozzle as an ink drop. As the bubble begins to collapse, the ink still in the channel between the nozzle and bubble starts to move towards the collapsing bubble, causing a volumetric contraction of the ink at the nozzle and resulting in the separation of the bulging ink as a droplet. The acceleration of the ink out of the nozzle while the bubble is growing provides the momentum and velocity of the droplet in a substantially straight line direction towards a recording medium, such as paper. The collection of ink on the nozzle-containing face of thermal ink-jet printheads causes all of the problems discussed above.
Ink jet printheads include an array of nozzles and may be formed out of silicon wafers using orientation dependent etching (ODE) techniques. The use of silicon wafers is advantageous because ODE techniques can form structures, such as nozzles, on silicon wafers in a highly precise manner. Moreover, these structures can be fabricated efficiently at low cost. The resulting nozzles are generally triangular in cross-section. Thermal ink jet printheads made by using the above-mentioned ODE techniques are typically comprised of a channel plate which contains a plurality of nozzle-defining channels located on a lower surface thereof bonded to a heater plate having a plurality of resistive heating elements formed on an upper surface thereof and arranged so that a heating element is located in each channel. The upper surface of the heater plate typically includes an insulative layer which is patterned to form recesses exposing the individual heating elements. This insulative layer is referred to as a "pit layer" and is sandwiched between the channel plate and heater plate so that the nozzle-containing face has three layers: the channel plate, the pit layer, and the heater plate. For examples of printheads employing this construction, see U.S. Pat. Nos. 4,774,530 to Hawkins and U.S. Pat. No. 4,829,324 to Drake et al, the disclosures of which are herein incorporated by reference. However, neither of these patents discloses non-wetting nozzle-defining channels having a step structure located between first and second ends thereof so that the channel inlets have a cross-sectional area greater than the cross-sectional area of the nozzle outlets.
In thermal ink jet technology, and in particular with the thermal ink jet printheads constructed from silicon channel plates and heater plates described above, the internal walls of each channel are wetting (wettable). The surface tension between ink in the channel and the wetting channel surfaces always drives the front face of the ink in the channel forward towards the outlet nozzle. Although a negative pressure is applied to the ink supply manifold (by locating the ink supply below the nozzles) to prevent ink from overflowing onto the front face of the chip, ink overflow during the refill process is still a common effect, causing wetting of the printhead front face and thus misdirection problems. Moreover, the front face of the ink is usually right at the outlet nozzle of each channel which results in the rapid drying of ink in the channels during periods of non-use.
The above-mentioned problems have been compensated for in the past by providing maintenance systems for wiping excess ink from the nozzle-containing face of printheads, as well as for capping and humidifying the nozzles during periods of non-use to prevent the drying of ink therein. Since the ink typically has a fast drying rate, it is difficult to prevent some drying from occurring even when a nozzle cap is provided. If the front face of the ink can somehow be held in the middle of each channel consistently, there will not be overflow and chip front face wetting during the refill surge of ink, or associated misdirection problems. Additionally, the ink drying process will be much slower. The maintenance system can then be simplified. Priming, wiping, purging, and even capping might not be necessary.
U.S. Pat. Nos. 4,728,392 and 4,801,955 to Miura et al disclose an electro-pneumatic ink jet printhead which includes a front nozzle having a front channel extending therethrough and aligned with the front channel. Inner and outer surfaces of the front nozzle as well as the surface of the front channel are coated with an ink repellant layer. The front face of the rear nozzle is also coated with an ink repellant layer. The front channel is forwardly tapered. Neither of these patents discloses non-wetting nozzle-defining channels having first ends attachable to an ink supply manifold, second ends defining outlet nozzles, and a step structure located between the first and second ends so that the first end has a cross-section larger in area than a cross-section of the second end, with a constant smaller cross-sectional area of the second end extending a distance from the step structure to the second end.
U.S. Pat. No. 4,422,086 to Miura et al discloses an ink jet printhead comprised of two identical chambers connected to first and second sections of a conduit, a channel for connecting the two chambers and a passageway connecting the channel with the printhead. The transverse cross section of the connecting channel and passageway is smaller than the cross section of the two chambers. However, the step structure is not located between the nozzle outlet and the means for expelling droplets from the nozzle. Additionally, the channels do not have non-wetting surfaces.
U.S. Pat. Nos. 4,555,062 and 4,583,690, both to You, and U.S. Pat. No. 4,643,948 to Diaz et al disclose nozzle-containing faces of ink jet printers which are coated with an anti-wetting agent. None of these patents discloses non-wetting nozzle-defining channels having a step structure located between first and second ends thereof so that the channel inlets have a cross-sectional area greater than a cross-sectional area of the nozzle outlets.
U.S. Pat. No. 4,751,532 to Fujimura et al discloses a thermal electrostatic ink jet recording head wherein thermal energy and an electrostatic field are applied to ink held between two plate members to cause the ink to be jetted out from an orifice defined by the plate members. Critical surface tensions must be satisfied to maintain a desired shape of the meniscus to provide good printing quality. Nozzle-containing surfaces of the plate members are treated to provide different surface tensions. The surfaces may be treated with a silicone-type or fluorocarbon-type resin. Fujimura et al requires that an area surrounding the nozzle remains adherent to liquid. This patent does not disclose non-wetting nozzle-defining channels, or a step structure between inlets and outlets of the channels so that the channel outlets have cross-sectional areas smaller than the channel inlets.
U.S. Pat. No. 4,623,906 to Chandrashekhar et al discloses a surface coating for ink jet nozzles. The coating includes a first layer of silicon nitride, an intermediate layer graded in composition, and a top-most layer of aluminum nitride. Chandrashekhar et al provide this structure to aid in adhering the low wettable, aluminum nitride layer to the nozzle-containing face which is made from glass or silicon. The channel of Chandrashekhar et al does not include a step structure between first and second ends thereof so that a constant smaller cross-sectional area of the second end extends a distance from the step structure to the second end.